The purpose of this research is to determine factors that regulate the activation of human naive T cells; in particular we are interested in the roles played by the NKG2D activation and LAIR-1 inhibitory receptors. Studies with T cell clones and activated T cell lines have shown that NKG2D is a co-stimulatory receptor for TCR mediated signals. We have purified CD8+ T cells and obtained a naive population by sorting for CD62L+ CD45RO- phenotype. Analyses showed that NKG2D serves as a co-stimulatory receptor for TCR induced Ca++ mobilization in freshly isolated naive CD8+ T cells. In addition, NKG2D co-stimulation results in effector cells that produce high amounts of IFNalpha and TNFalpha. The responses are equivalent or sometimes more than that seen for CD28 co-stimulation in these cells. The expression of the NKG2D ligands MICA, MICB and ULBP-1, -2, and -3 was observed in the proliferating cells, which we speculate accounts for the observed down-regulation of NKG2D in the activated cells. The addition of the homeostatic cytokines IL-7 and IL-15 to the culture medium not only enhanced proliferation but counteracted the down-regulation of NKG2D. These results indicate that NKG2D is an important co-stimulatory molecule in human naive CD8+ T cells and its expression is maintained by the presence of IL-7 and IL-15. LAIR-1 is constitutively expressed on the majority of human mononuclear leukocytes and functions as an inhibitory receptor on human NK cells, B cells, macrophages, and dendritic cells. Its expression by T cells and the role that it plays in these cells has not been welll characterized. We show that, in freshly isolated peripheral blood cells, LAIR-1 is differentially expressed in T cell subsets. CD8 T cells express higher levels of LAIR-1 than CD4. Naive CD4 and CD8 T cells express higher levels of LAIR-1 than memory T cells. Activation of peripheral blood T cells by cross-linking anti-CD3 mAb increases the cell surface expression of LAIR-1 in proliferating cells. Most importantly, we showed that LAIR-1 is a potent inhibitor of TCR mediated activation both in cultured and freshly isolated CD4 and CD8 T cells. We have also determined that LAIR-1 is expressed by mast cells and are in the process of determining if ligation of this inhibitory receptor can be used to control allergic reactions.